The present invention relates generally to reading the data content of non-volatile and other memory devices, and, more particularly, to using information on the distribution of program levels of a memory cell populations to more accurately read the content of degraded distributions.
As flash and other memory devices migrate to smaller geometries, the influence of a number of phenomena that negatively impact the robustness of data storage increases. Included in these factors are over-programming, read and program disturb, and data retention issues. These problems are often further aggravated as the number of states per cell is increased and as the operating window of stored threshold voltages shrinks. These factors are generally accounted for in the design phase of the memory devices through various tradeoffs that can be made within the design. These tradeoffs may increase or decrease the influence of one or the other of these factors, and/or tradeoff some of these factors against others, such as performance, endurance, reliability, and so on. In addition to tradeoffs within the memory design, there are a number of system-level mechanisms that may be incorporated to compensate for these phenomena, where needed, to achieve product-level specifications. These system mechanisms include ECC, wear-leveling, data refresh (or “Scrub”), and read margining (or “Heroic Recovery”), such as are discussed in U.S. Pat. Nos. 7,012,835, 6,151,246 and, especially, U.S. Pat. No. 5,657,332.
The above phenomena generally have the impact of affecting the distribution of cell voltage thresholds, either during programming, during subsequent memory operations, or over time, and they generally have a larger impact in multi-state memory storage relative to binary memory storage. The impact is typically to spread the voltage threshold levels of a given memory state within a population of cells, and, in some cases, to shift cell threshold levels such that they read in an erroneous state under normal read conditions, in which case the data bits for those cells become erroneous. As memories having smaller geometries become integrated into storage products, it is expected that the memory-level tradeoffs required to overcome the anticipated memory phenomena will make it difficult to achieve the required product-level specifications. Consequently, improvements to these devices will be required.